1. Field of the Invention
The present invention relates to a data transmission system, and more particularly, to an integrating receiver having an adaptive decision feedback equalizer function capable of simultaneously removing an inter-symbol interference and high frequency noises and a system having the same.
2. Description of the Related Art
As a dynamic random access memory (DRAM) data transmission method, there are a multi drop channel method of simultaneously connecting a single signal line with several chips in order to increase a transmission data capacity, and a single ended method of reducing the number of signal lines and pins.
In the multi drop channel method, as shown in FIG. 1, a single signal line 110 is connected to several DRAM chips 101 to 10n. In input pins of the DRAM chips 101 to 10n, a parasitic resistance, a parasitic inductance, and a parasitic capacitance exist. Due to the parasitic components, in the multi drop method, a signal attenuation occurs, so that a channel frequency band decreases. This works as an inter-symbol interference (ISI) in a high frequency signal transmission, so that a voltage margin and a time margin of a transmitted signal may decrease. In general, in order to remove the ISI, an equalizer is widely used.
FIG. 2 is a view showing signal response characteristics in the multi drop channel method. Referring to FIG. 2, when an input pulse with a pulse width T is applied to a channel having a limited bandwidth, since the channel bandwidth is limited, a response signal Vo in the channel cannot arrive at a destination within a time T. When a time is 2 T and 3 T, an influence of the response signal Vo remains. The remaining signal influences on a next period, so that the ISI also occurs.
FIG. 3 is a view for explaining a decision feedback equalizer (DFE) that is a kind of receiving terminal equalizer. Referring to FIG. 3, a DFE 300 includes an adder 310, a decider 320, and a feedback loop 330. The DFE 300 can remove an ISI that occurs in advance in a current input signal by using values ŷ[n−1] and ŷ[n−2] of data decided before a period and before two periods provided by the feedback loop 330. This can be represented as the following Equation 1.Y(nT)=VIN(nT)−a1●ŷ[n−1]−a2ŷ[n−2]  [Equation 1]
where a1 and a2 denote equalizer coefficients that represent an amount of ISI to be removed, and VIN(nT) denotes a receiving terminal input signal. Y(nT) denotes an output voltage of an equalizer from which the ISI is removed.
The DFE 300 has an advantage in that the DFE 300 can remove the ISI without amplifying high frequency noises of the input signal. However, since a maximum signal width of the input signal is attenuated and applied to the receiving terminal, there is a problem in that although the ISI is removed, a signal-to-noise ratio (SNR) decreases.
In order to use the equalizer, determining equalizer coefficients is most important. The equalizer coefficients may be changed according to characteristics of the channel and characteristics of a chip. In addition, the equalizer coefficients may also be changed according to processes of the chip, a voltage, and a change in temperature. In consideration of the change factors, precise equalizer coefficients can be determined. In general, a method of controlling an equalizer coefficient in order to have a maximum margin by using an input signal of the receiving terminal is widely used. This method is used for an adaptive equalizer.
In the single ended transmission method, in order to decide data in the receiving terminal, the data is compared to a reference voltage having an intermediate voltage between maximum and minimum voltage values of the data, so that a value of the data is decided. In this method, high frequency noises occur heavily as compared with a differential transmission method, so that there is a problem in that the signal-to-noise ratio (SNR) further decreases.
A first reason why the high frequency noises occur in the single ended transmission method is that a parasitic capacitance value between a reference voltage line and a common ground line and a parasitic capacitance value between the signal line and the common ground line are different such that high frequency noises that occur in the common ground line are differently applied to the signal line and the reference voltage line from each other. A second reason is that reflected waves and crosstalk that occur in the signal line occur as the high frequency noises.
As a method of reducing an influence of the high frequency noises that occur in the receiving terminal, there are a method of deciding a value of the data by extracting data several times during a period, and a method of deciding the value of the data by using an analog receiving circuit to perform an integration a value of the data during a period.
FIG. 4 is a view for explaining a conventional integrating receiver circuit. When the integrating receiver circuit 400 performs an integration function, an output voltage can be obtained by the following Equation 2.
                              Δ          ⁢                                          ⁢                      V            out                          =                              1                          2              ⁢              C                                ⁢                                    g              m                        ·                          ∫                                                (                                                                                    V                        IN                                            ⁡                                              (                        t                        )                                                              -                                          V                      ref                                                        )                                ⁢                                  ⅆ                  t                                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
where, C is a capacitance value in an output of an integrator, and gm is a trans-conductance value of an input transistor. The integrating receiver circuit 400 reduces a probability of wrong decision of data so as to increase a signal-to-noise ratio (SNR) of a receiver.
As an input/output data transmission speed of the DRAM increases, the ISI and high frequency noises of the receiving terminal occur more heavily in the aforementioned multi drop channel method, so that there is a limitation to the transmission speed. Channel ISI occurs by a limitation to a frequency of the channel, and the high frequency noises of the receiving terminals locally occur in a receiving chip, so that the two noises simultaneously occur.
However, the equalizer is vulnerable to the high frequency noises, and the integrating receiver is vulnerable to low frequency noises such as the ISI, so that applying only the equalizer or only the integrating receiver to the DRAM is inefficient to achieve a maximum performance.
Therefore, in order to simultaneously remove the two speed limitation factors, a new receiver implementing the equalizer and the integrating receiver as a single circuit is needed. In addition, a development of a method of obtaining an equalizer coefficient that can be applied to the new receiver may cause an improvement in the input/output data transmission speed of the DRAM.